Representative silylated aldophosphamide, isocyanate, phosphorimidate and sulfine with potential CNS antitumor activity as well as their l4C-labelled analogs will be synthesized. The hydrolytic stability of these compounds will be studied in vitro in order to establish the ease of removal of the protective silyl group. Brain uptake studies of the l4C-labelled compounds will be undertaken following the Oldendorf procedure in order to provide information on the drug's ability to cross the blood-brain barrier. If the measurements will indicate high lipophilicity (as compared with CCNU and BCNU) and proper rate of release of the moiety with potential antitumor activity from the prodrugs, additional derivatives will be synthesized.